The present invention relates to atomic force microscopy, and more particularly to atomic force microscopy scanning methods for tip orientation determination and adjustment.
Atomic force microscopy (AFM) probes are often used to evaluate and measure features on a semiconductor product as the semiconductor product is being developed or fabricated into an integrated circuit device, for example. Conventional AFM probes typically include a silicon cantilever beam with a silicon tip (xe2x80x9cAFM tipxe2x80x9d) extending perpendicular to or at a slight angle (e.g., 10 degrees) relative to the cantilever beam. The AFM tip is often formed into a long and thin rod. The AFM tip is often etched to form a sharp apex small enough to fit into a deep feature. There are several high aspect ratio tips on the market made for imaging and measuring deep narrow features. Some high aspect ratio tips are made using a focused ion beam to machine the silicon tip into a long thin rod with an aspect ratio between about 7:1 and 10:1. Hence, an AFM tip with a 10:1 aspect ratio (i.e., length:diameter) may be able to reach 1000 nm into a 100 nm diameter trench. Other high aspect ratio tips may be formed using electron beam deposition (e.g., EBD tips) or may be carbon nanotubes with a diameter between about 10 nm and 80 nm, for example.
As technology progresses, the features of integrated circuits typically become smaller, and in some cases, deeper. Thus, the demands on the size and precision of movement of AFM probe tips tends to increase as well. The depth that an AFM tip can reach into a deep feature depends on the angle or orientation of the tip relative to the deep feature sidewalls. If the center line of the AFM tip is perpendicular to the wafer surface or parallel to the deep feature sidewalls, then the AFM tip can reach deepest a center region of the deep region. If a left side or left face of the AFM tip is parallel with or inverted relative to a left sidewall of the deep feature, then the AFM tip can reach deepest at a left side of the deep feature along its left sidewall. Such scan may be desirable to obtain an AFM image of the left sidewall or to measure the depth of the deep feature at the bottom left corner. However, without knowing the actual AFM tip orientation and without being able to control the actual tip orientation, the AFM image results may be misleading. Due to the size of the deep features being measured for semiconductor devices relative to the size of the AFM tip, the actual AFM tip orientation must be known and controlled to obtain accurate AFM images of the deep features. Hence, there is a need for a way to measure, adjust, and control the actual AFM tip orientation for a given scanner head and AFM tip setup.
The problems and needs outlined above are addressed by embodiments of the present invention. In accordance with one aspect of the present invention, a method of calibrating a scanner head of an atomic force microscopy (AFM) machine is provided. The method includes the following steps, the order of which may vary. First, a calibration sample is loaded into the AFM machine. Second, deep features formed in the sample are scanned along a first line in a first direction with an AFM tip that is attached to the scanner head. Third, a first tip angle measurement is determined for the AFM tip relative to the sample along the first line for at least two of the deep features scanned along the first line. Fourth, deep features formed in the sample are scanned along a second line in a second direction with the AFM tip. The second direction differs from the first direction. Fifth, a second tip angle measurement is determined for the AFM tip relative to the sample along the second line for at least two of the deep features scanned along the second line. Sixth, a first function is determined which corresponds to the first tip angle measurements versus position along the first line. Seventh, a second function is determined which corresponds to the second tip angle measurements versus position along the second line. Eighth, the first and second functions are stored as the arc functions for the scanner head.
In accordance with another aspect of the present invention, a method of measuring an actual tip angle for an AFM tip is provided. This method includes the following steps, the order of which may vary. First, a sample having at least one deep feature formed therein is provided. The deep feature has vertical sidewalls. Second, the deep feature is scanned with the AFM tip. Third, a cross-section image of deep feature scan data is analyzed to determine a slope of a left sidewall of the cross-section AFM image. Fourth, the cross-section image of deep feature scan data is analyzed to determine a slope of a right sidewall of the cross-section image. Fifth, the actual tip angle of the AFM tip is determined at a position where the deep feature was scanned based on the left and right sidewall slopes.
In accordance with yet another aspect of the present invention, a method of calibrating an AFM tip is provided. This method includes the following steps, the order of which may vary. First, a calibration sample is loaded into an AFM machine. The sample has deep features formed therein. Second, a scanner head is moved to an initial position. The AFM tip is attached to the end of the scanner head. Third, one of the deep features is scanned at the initial position. Fourth, a first tip angle measurement is determined for the AFM tip relative to the sample in a first direction. Fifth, a second tip angle measurement is determined for the AFM tip relative to the sample in a second direction. The second direction differs from the first direction. Sixth, a first position offset in the first direction is determined for performing a first scan. Seventh, a second position offset in the second direction is determined for performing the first scan. The first scan may be a center scan for scanning a central portion of one of the deep features so that the first and second position offsets correspond to a position coordinate where the AFM tip is substantially perpendicular to a surface of the sample in both the first and second directions. The first scan may be a left scan for scanning a left side portion of one of the deep features so that the first and second position offsets correspond to a position coordinate where a left face of the AFM tip is substantially perpendicular to a surface of the sample in the first direction. And, the first scan may be a right scan for scanning a right side portion of one of the deep features so that the first and second position offsets correspond to a position coordinate where a right face of the AFM tip is substantially parallel with a right sidewall of the deep feature to be scanned for the first scan. The first position offset may be verified (if needed or desired) by performing a test scan and measuring the tip angle with the scanner head at the first position offset.
In accordance with yet another aspect of the present invention, a method of obtaining a partial deep feature AFM image is provided. This method includes the following steps, the order of which may vary. First, a sample is loaded into an AFM scanner machine. The sample has deep features formed therein, and the AFM scanner machine has a scanner head with an AFM tip attached thereto. Second, a desired deep feature image portion to be scanned is selected from choices including a center portion, a left portion, and a right portion. Third, if the center portion is selected for the deep feature image portion desired, the scanner head is moved to a first position where the AFM tip is substantially perpendicular to a surface of the sample. If the left portion is selected for the deep feature image portion desired, the scanner head is moved to a second position where a left face of the AFM tip will be substantially parallel with a left sidewall of one of the deep features at the second position. If the right portion is selected for the deep feature image portion desired, the scanner head is moved to a third position where a right face of the AFM tip will be substantially parallel with a right sidewall of one of the deep features at the third position. Fourth, one of the deep features is scanned at the current scanner head position corresponding to the selected portion to obtain a cross-section AFM image of the deep feature focusing on the selected portion. The AFM image may be analyzed to obtain a depth measurement for the selected portion of the deep feature scanned.
In accordance with a further aspect of the present invention, a method of obtaining a composite deep feature AFM image is provided. This method includes the following steps, the order of which may vary. First, a sample is loaded into an AFM scanner machine. The sample has deep features formed therein. The AFM scanner machine has a scanner head with an AFM tip attached thereto. Second, the scanner head is moved to a first position where the AFM tip is substantially perpendicular to a surface of the sample. Third, a first deep feature at the first position is scanned to obtain a first cross-section AFM image of the first deep feature focusing on a center portion of the first deep feature. Fourth, the scanner head is moved to a second position where a left face of the AFM tip will be substantially parallel with a left sidewall of a second deep feature at the second position. Fifth, the second deep feature at the second position is scanned to obtain a second cross-section AFM image of the second deep feature focusing on a left portion of the second deep feature. Sixth, the scanner head is moved to a third position where a right face of the AFM tip will be substantially parallel with a right sidewall of a third deep feature at the third position. Seventh, the third deep feature at the third position is scanned to obtain a third cross-section AFM image of the third deep feature focusing on a right portion of the third deep feature. Eighth, the center portion of the first image is combined with the left portion of the second image and with the right portion of the third image to form the composite AFM image.
The foregoing has outlined rather broadly the features of the present invention so that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those of ordinary skill in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those of ordinary skill in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.